1. Field of the Invention
The present invention relates to a method of monitoring the supply of looper thread, in particular, a method by which information as to the existing supply of thread on the bobbin of looper thread ready for sewing can be obtained at any time.
2. Description of the Related Prior Art
A thread-monitoring device of related interest is known from German Unexamined Application for Patent DE-OS 36 25 630 A1. The known thread-monitoring device consists essentially of a sensor unit, a data processing unit, and a signal device. The sensor unit has a source of light, for instance a light transmitter, as well as a light receiver, for instance a photodetector. A beam of light which is sent out by the source of light is directed onto a bobbin which is provided to contain the supply of looper thread and which is incorporated in a bobbin case, also referred to as bobbin-housing upper part, of a rotating lock-stitch looper. Said beam of light passes in this connection through an opening provided in the bobbin case and thus strikes the outside of a front flange, the outside being provided with black light-absorbing and white light-reflecting markings. Said bobbin briefly carries out a turning movement within a period of time during which the amount of thread required for the formation of a lock-stitch is withdrawn from the supply of looper thread present on the bobbin. Upon this turning movement of the bobbin, the sensor unit scans the alternate black and white markings, as a result of which electric pulses are produced. These pulses are fed to a data processing unit and a signal device, whereby the supply of looper thread present on the bobbin is monitored. This monitoring is based on an evaluation of the continuously changing conditions of reflection, which permit corresponding conclusions as to the rotation of the bobbin and the existing supply of looper thread on the bobbin. When a predetermined residual amount of thread still present on the bobbin is reached, the signal device gives off a visual and/or acoustic warning signal.
The disadvantage of the known monitoring device is that the said monitoring refers only to the conditions "bobbin full; bobbin empty; thread tear", while an indication as to the condition with respect to the supply of thread at hand at the time is not possible.
From French Unexamined Application No. 2 600 085, it is furthermore known to provide scannable reflection surfaces on the flange of the bobbin. Upon opto-electronic scanning of the reflection surfaces, pulses are produced which represent a measure of the supply of looper thread present on the bobbin. Since the solution proposed does not provide for any possibility of storing the data defining the supply of looper thread, the bobbin-specific data describing the supply of looper thread are lost as soon as the bobbin in question is removed from the looper and separated from the sewing machine. From said application, to be sure, a detection of the pulses via a marking on the outside of a flange of the bobbin, both upon the winding and upon the sewing, is known, but, aside from this, no indication can be obtained by which it is possible to note the existing state of fullness of the bobbin by means of the pulses detected and the geometrical data of said bobbin.